In recent years, an ink jet recording system has been employed in a variety of printing fields such as photography, various types of printing, and special printing such as marking and color filters, from the viewpoints that it is capable of image formation in a simple manner at low cost.
As an ink jet ink used in such an ink jet recording system, there are various types of ink jet inks such as an aqueous ink jet ink containing water as a main solvent, an oily ink jet ink mainly containing a non-volatile solvent which does not volatilize at room temperature and containing substantially no water, a non-aqueous ink jet ink mainly containing a solvent volatile at room temperature and containing substantially no water, and a hot melt ink jet ink in which ink, being solid at room temperature, is heat-melted for printing, as well as an actinic energy radiation curable ink jet ink which is cured with actinic energy radiation such as light after printing. These ink jet inks may be used based on the purposes. Among these, the actinic energy radiation curable ink jet ink is characterized by being printed on various printing media due to rapid curability, whereby it has attracted much attention as a next-generation ink jet ink, which replaces the aqueous ink jet ink, the oily ink jet ink, and the non-aqueous ink jet ink requiring a large drying load, each having limited recording media, and further, is expected to expand in uses.
In the related art, the actinic energy radiation curable ink jet inks are mainly classified into a radically polymerizable type ink jet ink and a cationically polymerizable type ink jet ink. Further, the radically polymerizable type ink jet ink features a wide selection of materials, whereby it has great flexibility for ink designing and has been widely researched and developed, resulting in being put into practical use. However, it is easily affected by polymerization inhibition caused by oxygen, and is not easy to handle from the viewpoints of, for example, generating unfavorable odor of polymerizable compounds (monomers) or having skin sensitivity. On the other hand, the cationically polymerizable type ink jet ink is not affected by polymerization inhibition caused by oxygen, but it has problems that it involves polymerization inhibition by an effect of humidity and has high cost due to little variation in materials.
Recently, for the purpose of satisfying both of stability of materials used, such as polymerizable compounds (monomers), polymerization initiators, and sensitizers, and practical curing sensitivity, there has been an attempt to apply a charge transfer complex polymerization reaction (abbreviated as a CT polymerization reaction, and see, for example, NPL 1) to an actinic energy radiation curable composition such as an ultraviolet ray curable composition (see, for example, PTLs 1, 2, and 3). The charge transfer complex polymerization reaction (CT polymerization reaction) is carried out by combining electron-rich monomers (donor-monomers) such as vinyl ether and electron-poor monomers (acceptor-monomers) such as maleimide, or combining monomers having low e values, such as vinyl ether and monomers having high e values, such as maleimide.
However, in a case where these charge transfer complex polymerization-based monomers are applied in actinic energy radiation curable ink jet inks, severe problems have been caused. That is, it is essential that the ink jet ink should be an ultra-low-viscosity liquid, as compared with a printing ink or composition for coating, usually irrespective of a piezo type, a thermal type, an antistatic system, or the like, since the ink jet ink cannot be stably discharged from the nozzles of an ink jet recording head. Further, if the viscosity of the ink changes to a great or small extent by the effects of storage for a long period of time or storage environment, the amount of the liquid droplets of the ink from the nozzles or the scattering speed of the liquid droplets of the ink are affected, and as a result, the precision of the printing is deteriorated, which leads to turbulence of the image. However, in the present situation, in the composition using CT polymerization-based compounds in the related art, the precision has not reached the level with respect to the storage stability of the ink jet ink, required for such an ink jet ink system.
In addition, by the investigation of the present inventors, it has been proved that sufficient strength cannot be ensured for a film formed from a CT polymerization type ink jet ink. In particular, in a case of using a light source having low illuminance, such as an LED, there is a tendency that the resistance to light, rain, or the like outdoors (weather resistance) is insufficient or the adhesion to a rigid substrate is insufficient. In a recent situation, there has been a desire for an actinic energy radiation curable ink which is cured with a smaller amount of light in the ink jet systems, and the CT polymerization type ink jet ink could not meet these requirements.